Method for the preparation of a sintered body of high-hardness high-chromium cast iron

ABSTRACT

Disclosed is a method for the preparation of a sintered body of a high-chromium cast iron of a specified chemical composition having greatly improved mechanical, abrasion-resistant and corrosion-resistant properties as compared with conventional cast bodies of the same cast iron. The inventive method comprises the steps of preparing a powder of the cast iron alloy by quenching solidification of a melt, e.g., by centrifugal spray atomization, and sintering the powder under compression by the discharge plasma sintering method.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an efficient method for the preparation of a sintered body of a high-chromium cast iron having an outstandingly high hardness, abrasion resistance, heat resistance and corrosion resistance as compared with bodies of the same high-chrome cast iron prepared by a conventional casting method so as to be useful as a part of crushers, grinding machines and the like as well as in thermal electric power plants, iron- and steel-making plants, cement-making plants and many other industrial facilities.

[0002] As is well known, high-chromium cast irons are generally excellent in abrasion resistance and heat resistance by virtue of the high content of a carbide phase of high hardness and also has high corrosion resistance not only at room temperature but also at high temperatures because a large amount of chromium forms a solid solution in the matrix phase of iron. These excellent properties of a high-chromium cast iron, in combination with the relatively low material cost, are fully utilized as a material of high-performance parts such as protective tubes, coal crushers, nozzles, impellers and the like in the industries of thermal electric power generation, cement manufacturing and others.

[0003] To be in compliance with the requirements in these industries in recent years for improving the productivity and economic merit more and more by accomplishing extension of the intervals or dispensability of the maintenance services of the plants, parts of the machines and plants made from a high-chromium cast iron by the conventional procedure are not quite satisfactory in respect of their abrasion resistance, heat resistance, corrosion resistance and other properties so that it is eagerly desired to develop a method for satisfying the requirements.

[0004] When improvement of the abrasion resistance is desired of a body made from a high-chromium cast iron, for example, a possible and effective way therefor is to have a content of carbon equal to or higher than the eutectic line in the iron/chromium/carbon ternary alloy system so as to increase precipitation of the high-hardness carbide phase of the composition of the M₇C₃ type. A body of such a hypereutectic composition of the alloy, however, usually exhibits remarkable anisotropy due to inhomogeneous crystallization of coarse carbide crystals along the direction of the heat flow or, namely, the direction of solidification unavoidably decreasing the mechanical properties including embrittlement. This is the reason for the fact that high-chromium cast irons of a hypereutectic composition have rarely come under practical applications heretofore.

SUMMARY OF THE INVENTION

[0005] The present invention accordingly has an object, under the above described circumstances in the prior art, to provide a novel and efficient method for the preparation of a high-hardness body of a high-chromium cast iron having outstandingly excellent abrasion resistance, heat resistance and corrosion resistance in combination and suitable as a material of parts in plants and machines which can be dispensed with the maintenance services or of which the intervals of the maintenance services can be substantially extended.

[0006] Thus, the present invention provides a method for the preparation of a high-hardness sintered body of a high-chromium cast iron which comprises the steps of:

[0007] (a) subjecting a melt of a ternary alloy of iron, chromium and carbon containing from 11 to 30 mass % of chromium and from 2.2 to 5.0 mass % of carbon, the balance substantially being iron, to a quenching solidification treatment at a cooling rate not lower than 10⁴° C./second to give particles of the ternary alloy; and

[0008] (b) subjecting the particles of the ternary alloy to a sintering treatment by the method of electric discharge plasma sintering under compression in the atmospheric air.

[0009] In particular, it is preferable that the quenching solidification of the alloy melt in step (a) is conducted by the method of centrifugal spray atomization.

BRIEF DESCRIPTION OF THE DRAWING

[0010] Photos A and B of FIG. 1 are each a microscopic photograph showing the metallographic structure of the sintered bodies of high-chromium cast irons of hypoeutectic and hypereutectic compositions, respectively, prepared in Example. Photos C and D of FIG. 1 are each a microscopic photograph showing the metallographic structure of the metal mold-cast bodies of the same hypoeutectic and hypereutectic compositions as in Photos A and B, respectively, prepared in Comparative Example.

[0011]FIG. 2 is a bar chart showing the Rockwell hardness of the shaped bodies at room temperature, in which the columns A, B, C and D correspond to Photos A, B, C and D, respectively, of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] As is described above, the method of the present invention comprises two essential steps (a) and (b), each of which is very unique as compared with the conventional powder metallurgical process including the steps of casting of an alloy melt into a mold to give an ingot, pulverization of the alloy ingot into fine particles, shaping the alloy particles by compression molding into a powder compact and sintering of the powder compact in a sintering furnace.

[0013] Namely, step (a) of the inventive method is for the preparation of alloy particles from a melt of a high-chromium cast iron of the specified composition, in which the alloy melt is subjected to quenching solidification or, preferably, by the method of centrifugal spray atomization directly into alloy particles at a cooling rate higher by at least 10⁵ times or sometimes by 10⁷ times than the cooling rate in the conventional casting method. The alloy particles prepared as above are then subjected to a discharge plasma sintering treatment under compression in the atmospheric air into a sintered body.

[0014] The above described inventive method is applicable not only to a hypoeutectic iron/chromium/carbon ternary alloy, of which the content of carbon is lower than the eutectic line, as a matter of course but also to a hypereutectic iron/chromium/carbon ternary alloy, of which the content of carbon is higher than the eutectic line. The metallographic texture of the thus obtained sintered body is much finer and much more uniform beyond comparable than that of the same cast iron body prepared by the casting method.

[0015] The chemical composition of the melt of the high-chromium cast iron used in the inventive method is limited such that the contents of chromium and carbon are in the ranges from 11 to 30 mass % and from 2.2 to 5.0 mass %, respectively, the balance being substantially iron with small amounts of unavoidable impurity elements.

[0016] The melt of the high-chromium cast iron is subjected in step (a) to a quenching solidification treatment at a cooling rate of at least 10⁴° C./second or, desirably, at least 10⁵° C./second. The cooling rate of the melt in the quenching solidification can be readily estimated according to a known procedure by utilizing the established relationship between the cooling rate and the metallographic structure of the solidified alloy. This quenching solidification treatment of the melt is conducted preferably by the centrifugal spray atomization method, in which the melt is ejected at a quenching disk rotating at a high velocity to be atomized by the centrifugal force into fine droplets, which are quenched by blowing of an inert gas to be solidified to give fine particles.

[0017] Alternatively, the so-called quenching roller method using a single quenching roller or twin quenching rollers is applicable to step (a) though less preferable than the centrifugal spray atomization method because the quenching roller method produces thin ribbons of the solidified alloy which are more bulky than powders and need an additional step of pulverization or grinding with an unavoidable risk of contamination from the pulverization or grinding machine.

[0018] The other advantages of the centrifugal spray atomizing method over other powder-making methods include: (1) the high cooling rate of the melt; (2) unnecessity of any carrier gas for spraying to ensure little gaseous occlusion in the particles; (3) little formation of oxidized surface film on the particles; (4) good sphericity of the particle configuration with little adhesion of satellite particles; and (5) independent controllability of the spraying conditions and quenching conditions to facilitate running of the process with stability.

[0019] The quench-solidified particles of the high-chromium cast iron are then subjected to sintering by the so-called discharge plasma sintering method or pulse-current sintering method at a temperature in the range from 1000 to 1200° C. under compression with a molding pressure of 10 to 50 MPa with application of a pulsewise direct-current power. When undertaken under adequate conditions, the sintering treatment can be completed within a very short time of a few minutes. In the process of discharge plasma sintering, as is usually accepted, plasma discharge is first generated between particles so as to activate the particle surfaces by the accumulation of the impact energy thereby in the form of heat or strain so as to expose clean surfaces by removing the gases and contaminants adsorbed on the particle surfaces and destroying the oxidized surface film having a thickness of several nanometers order. Thereafter, Joule's heat is generated at the contacting points between particles by passing an electric current through the powder under compression so that the particles are bonded together to effect sintering giving a sintered body. This discharge plasma sintering treatment can be conducted in the atmospheric air. This process is effective for the consolidation of the particles in the non-equilibrium conditions.

[0020] The sintered body of the high-chromium cast iron obtained in the above described manner is characterized by the very fine and uniform metallographic texture, which is absolutely free from anisotropy in contrast to the shaped body prepared by casting of a melt more or less having anisotropy of the texture.

[0021] Accordingly, the sintered body of the high-chromium cast iron prepared by the inventive method has a Rockwell hardness greatly exceeding that of the bodies prepared by casting along with excellent abrasion resistance as a conesquence of strengthening by the fine texture.

[0022] In the sintered body of the high-chromium cast iron prepared by the inventive method, the element of chromium forms a solid solution in the iron matrix and plays a very important role to increase the corrosion resistance of the iron matrix and to form a primary crystals of carbide (Fe,Cr)₇C₃. The content of chromium in the ternary alloy, however, is limited to the range of from 11 to 30 mass % because, when too small, the above mentioned advantageous effects cannot be fully exhibited while, when too large, a decrease is resulted in the hardness of the alloy.

[0023] On the other hand, carbon is an indispensable element to form the phase of carbides with chromium and the content of carbon in the alloy is limited to the range of from 2.2 to 5.0 mass % because, when too small, the amount of the carbides formed therefrom is naturally so small that a decrease is caused in the hardness and abrasion resistance of the sintered body while, when too large, the amount of the carbides formed from carbon is so large that the sintered body suffers a decrease in the toughness.

[0024] In the following, the method of the present invention is described in more detail by way of an Example and a Comparative Example, which, however, never limit the scope of the invention in any way.

EXAMPLE

[0025] Quench-solidified powders were prepared by the centrifugal spray atomizing method from a melt of a hypoeutectic high-chromium cast iron of the chemical composition of Fe-25.3Cr-2.60C and a melt of a hypereutectic high-chromium cast iron of the chemical composition of Fe-24.4Cr-4.74C. The cooling rate in the quenching solidification was estimated to be in the range from 10⁵ to 10⁴° C./second.

[0026] After particle size classification to collect the particles passing a mesh screen of 177 μm mesh opening, each of the powders was subjected to a discharge plasma sintering treatment in a graphite mold for 3 minutes under a molding pressure of 32 MPa with application of an on-off pulsed electric voltage of 33 milliseconds pulse width. The sintering temperature was 1140° C. for the powder of the hypoeutectic composition and 1100° C. for the powder of the hypereutectic composition. The time taken for arriving at the sintering temperature was about 300 seconds.

[0027] Photos A and B of FIG. 1 are each a microscopic photograph showing the metallographic structure of the thus obtained sintered bodies of the hypoeutectic alloy and hypereutectic alloy, respectively. As is understood from these photographs, each of these sintered bodies has a uniform texture with extremely fine dispersion of the iron matrix phase and the carbide phase.

[0028]FIG. 2 is a bar chart showing the Rockwell hardness HRC of these sintered bodies, of which the columns A and B are for the sintered bodies having the hypoeutectic and hypereutectic compositions of which the Rockwell hardness was 63 HRC and 68 HRC, respectively.

COMPARATIVE EXAMPLE

[0029] Metal mold-cast bodies were prepared by casting of the melts of the same hypoeutectic and hypereutectic high-chromium cast irons used in the above described Example. As a consequence of the use of a metal mold instead of a sand mold conventionally employed in casting of cast irons, the solidification rate of the melt could be estimated to be larger than in a sand mold leading to an improvement of the mechanical properties of the cast body.

[0030] Photos C and D of FIG. 1 are each a microscopic photograph showing the metallographic structure of the thus obtained cast bodies of the hypoeutectic alloy and hypereutectic alloy, respectively. In the cast body of the hypoeutectic alloy, Photo C indicates primary crystallization of coarse dendritic austenite crystals, i.e. γ-phase, with intervention of coarse eutectic crystals of the γ-phase and carbide in the interstices. Photo D indicates that the cast body of the hypereutectic alloy consists of coarse primary crystals of the carbide and eutectic crystals of the γ-phase and carbide. In each of these cast bodies, the texture has developed in the direction of solidification or, namely, in the direction of the heat flow.

[0031] In FIG. 2, columns C and D show the Rockwell hardness of 50 HRC and 57 HRC of the hypoeutectic and hypereutectic cast bodies, respectively. It is evident that the sintered bodies of the high-chromium cast iron prepared according to the inventive method are very superior in the mechanical properties as compared with the cast bodies of the same alloy composition. 

What is claimed is:
 1. A method for the preparation of a sintered body of a high-chromium cast iron which comprises the steps of: (a) subjecting a melt of a ternary alloy of iron, chromium and carbon containing from 11 to 30 mass % of chromium and from 2.2 to 5.0 mass % of carbon, the balance substantially being iron, to a quenching solidification treatment at a cooling rate not lower than 10^(4°) C./second to give particles of the ternary alloy; and (b) subjecting the particles of the ternary alloy to a sintering treatment under compression by the electric discharge plasma sintering method.
 2. The method for the preparation of a sintered body of a high-chromium cast iron as claimed in claim 1 in which the quenching solidification of the alloy melt in step (a) is conducted by the centrifugal spray atomization method.
 3. The method for the preparation of a sintered body of a high-chromium cast iron as claimed in claim 1 in which the pressure of compression in step (b) is in the range from 10 to 50 MPa.
 4. The method for the preparation of a sintered body of a high-chromium cast iron as claimed in claim 1 in which the temperature of sintering in step (b) is in the range from 1000 to 1200° C.
 5. The method for the preparation of a sintered body of a high-chromium cast iron as claimed in claim 1 in which the sintering treatment in step (b) is conducted in the atmospheric air. 